i) Field of the Invention
This invention relates to a method and apparatus for determining the thickness of bones of the skull in vivo.
ii) Description of Prior Art
Cranial bone grafts have become the substrate of choice in rehabilitation and reconstruction of the craniomaxillofacial skeleton; and the calvarium is generally accepted as the best donor site. Advantages of the calvarium as a donor site for bone reconstructive surgery include accessibility, the inherent contour and abundance of harvestable bone, improved graft volume survival and the inconspicuousness of the donor site.
Knowledge of calvarial thickness at the donor site would be of significant assistance to surgeons harvesting calvarial grafts.
Computerized tomography provides a reasonable estimate of cranial thickness, however, this imaging modality lacks precision when extrapolating for in situ assessment.
It has been demonstrated that calvarial bone thickness is subject to regional variation and thus far accurate methods for the in situ or in vivo measurement of skull thickness are not available.
Previously it has been suggested that 6 mm of parietal bone thickness is the threshold for safe in situ calvarial harvesting, while others have suggested that a 2 mm thickness margin of the diploic space should be the limiting factor in obtaining a safe separation plane in split cranial harvests.
Despite the limited number and nature of predictive studies on the assessment of skull thickness, the importance of recognizing the variation between minimum and maximum skull thickness at a particular site is established. This variation can approach 8 mm. The variations and lack of predictability of bone thickness based on tables and charts of average data places predictability well beyond the margin of safety required in the context of consistently harvesting cranial bone without compromising the patient.
Knowledge of the thickness of the maxilla in preparation for cranial facial surgery or dental surgery would also be of value, so as to avoid penetration through the maxilla and potential damage or infection of underlying organs.
Ultrasound has been used in other environments in evaluating thickness or changes in thickness of an industrial article such as result from disintegration or flaking or separation of parts of the article. U.S. Pat. No. 5,351,544 describes a measuring apparatus which employs high frequency ultrasound to determine the thickness and/or flaking state of a specimen along its depth. U.S. Pat. No. 5,440,929 describes an ultrasonic device for measuring thickness of a bottom plate of an oil storage tank. U.S. Pat. No. 3,985,022 describes a technique for ultrasonic measurement of a work piece. U.S. Pat. No. 5,009,103 describes an ultrasonic thickness measuring method and apparatus. U.S. Pat. No. 5,585,563 describes a method for determining the thickness of a specimen such as an optical lens without mechanical contact.
All of these prior methods employing ultrasound are directed to evaluating articles of manufacture where the opposed surfaces of the article are accessible or can be located. None of these prior Patents is concerned with measurement or evaluation of bone or the like, in vivo.
The bones of the skull include the cranium bones and the facial bones, for example, the calvarial and the maxilla.